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I have a 502 roller motor with Aluminum heads that was dynoed at 560 hp at 5700 rpm. Boat ran good all summer long, but during the winter got the speed bug and installed a Vortec supercharger.
I'm getting some oil out of the single K&N breather that is in the Valve cover (is this enough crankcase ventilation?) I was told to do a leak down on the motor but I'm not sure what is acceptable, I was told max of 12% :confused:
I'd go 20% leakage [past the rings] before I would freshen the engine. Make sure your leakage isn't passed thru the valves or gaskets before tearing down the engine completly, maybe all it needs is a valve job and/or head gaskets.
You don't have to rebuild the motor if you have a lot of leak down. It just won't perform at it's best. Your oil will get dirty quicker and you won't make as much power. I would be more concerned that all cylinders leak down the same than with the leak down number. For what it's worth, I have a naturally aspirated 502 and it leaks some oil out the breathers too.
There's two things that cause oil out the breather.
1. Not enough breather or breather directly over rocker stream (or both)
2. Too much crankcase pressure usually from going by the rings.
If you have only one KN I'd suspect #1 first.
Assuming a mid ninety per cent leak down on the initial dyno testing, figure the horsepower lost with an eighty eight per cent engine.
The way I look at it, 12% leak down on a V8 means your almost running on seven cylinders.
The choice is yours.
Finally received the leak down tester. and found time to work on the boat.
Ran a compression check first. I figured these numbers were not to bad.
Results:
#1. 135psi #2. 135psi
#3. 125psi #4. 140psi
#5. 140psi #6. 130psi
#7. 135psi #8. 135psi
Then I ran the leak down according to the instructions.
Results:
#1. 32% exhaust leak. #2. 6% Good
#3. 32% exhaust leak. #4. 17% Gurgle in motor
#5. 8% Good #6. 28% Gurgle in motor
#7. 6% Good #8. 22% Gurgle in motor
I'm thinking couple of burned exhaust valves and hopefully a bad head gasket. (New Iconel Valves for sure.)
Any other thoughts would be appreciated?
With all this the last time on the water at Havasu it hit 80 on the gps
Thanks Clint
More proof a compression tester doesn't tell much.
Leak the motor at TDC each cylinder. Rock the piston a little during leak test to help the ring seat. Listen at the exhaust and the intake to determine where the cylinder is leaking. You can also remove all the rockers and leak the cylinder while checking for air leakeage into the cylinder next door. Many times head gaskets leak between cylinders.
Sometimes rapping on a valve with a soft hammer that's seeping will reseat the valve slightly or blow debris from the valve seat.
If you decide to have a valve job done (which sounds like you may) be sure to leak test the motor immediately after you install the heads. You might be surprised at many who do valve jobs that don't seal.